Most of electronic machines produced in recent years have many semiconductor devices and many circuit substrates. Patterns for wiring and connection terminals in these semiconductor devices and circuit substrates are generally produced by a processing technique in which a thin, fine resist pattern of a photosensitive resin (to be referred to as "photoresist" hereinafter) is formed on a substrate and then, the substrate is etched.
The formation of a thin, fine resist pattern of a photoresist is generally carried out by a method In which the film surface of the photoresist formed on a substrate is exposed to light through a photomask having a light-shielding pattern.
In the above method, generally, the film thickness of a photoresist formed on a substrate is 0.5 to 3 .mu.m when the photoresist is formed by coating a solution of the photoresist, and the film thickness is less than 50 .mu.m even when a film-like photoresist called a dry film is stacked. Therefore, the film thickness of a fine resist pattern formed by the above method is inevitably less than 50 .mu.m.
Meanwhile, for example, in the production of a printed circuit substrate on which a semiconductor device is to be mounted directly and a mounting wiring substrate typified by TAB (tape automated bonding) for connecting lead frames or a semiconductor device to a printed circuit substrate, it is being desired to establish a technique for forming a highly accurate resist pattern having, for example, a thickness of 50 to 500 .mu.m.
In the above method, however, when a photoresist formed on a substrate has a film thickness of 50 .mu.m or more, it is required to increase exposure energy so that the photoresist is exposed up to its bottom in exposing the photoresist to light through a photomask. However, when the exposure energy is increased, ultraviolet light reaches even a place below a pattern of a light-shielding film, and no proper pattern can be obtained.